1. Field of the Invention
The invention relates to a charge-pump circuit for the PLL (Phase Lock Loop), and more particularly to a feedback charge-share suppressing charge-pump circuit.
2. Description of the Related Art
When generating required system clocks, it is necessary to input referenced clocks in phase lock loops (PLL) or clock synthesizers. FIG. 1 is a block diagram illustrating a conventional phase lock loop system. The phase lock loop system 100 comprises a phase frequency detector 102, a charge-pump circuit 104, a voltage-controlled oscillator 106, a divider 108 and a loop filter 110. The phase lock loop system 100 receives a referenced clock Fin. The charge-pump circuit 104 drives the voltage-controlled oscillator 106 to generate a required clock Fout at a signal generated by the phase frequency detector 102. The required clock Fout is fed back to the phase frequency detector 102 through the divider 108. The loop filter 110 is coupled to an output terminal of the charge-pump circuit 104. A detailed diagram of the conventional charge-pump circuit 104 and loop filter 110 is shown in FIG. 2a. 
FIG. 2a is a circuit diagram illustrating a conventional charge-pump circuit and loop filter as shown in FIG. 1. As shown in FIG. 2a, the charge-pump circuit 104 comprises current sources 202 and 204, switches 206 and 208, and a capacitor 210. The loop filter 110 is composed of a capacitor 212. The loop filter 110 is coupled to an output terminal of the charge-pump circuit 104. When the switch 206 is in xe2x80x9cOnxe2x80x9d state, the loop filter 110 is charged by the current sources 202. When the switch 208 is in xe2x80x9cOnxe2x80x9d state, the loop filter 110 supplies the stored power to the switch 208 and the current source 204. A charge-share problem occurs in the charge-pump circuit 104. FIG. 2b is a diagram illustrating the charge-share problem in the charge-pump circuit shown in FIG. 2a. The X axis is time, in units of seconds (s). The Y axis is the output voltage Vc, in units of volts (V). Line 22 shows a normal voltage curve. Dotted line 24 shows a curve of the voltage affected by the charge-share problem when the switch 206 changes from xe2x80x9cOffxe2x80x9d state to xe2x80x9cOnxe2x80x9d state. Because of the charge-share problem, the signal driving the controlled oscillator 106 is incorrect. Thus, the phase lock loop system cannot generate the required clock.
To preventing the charge-share problem, Ian A. Young, Jeffrey K. Greason, and Keng L. Wang provide a charge-pump circuit for charge-share suppression with an operational amplifier (referring xe2x80x9cA PLL Clock Generator with 5 to 110 MHz of Lock Range for Microprocessors,xe2x80x9d IEEE J. Solid State Circuits, vol. 27, pp. 1599-1607 November 1992). FIG. 3 is a circuit diagram illustrating the conventional charge-pump circuit for charge-share suppression with the operational amplifier OP1. As shown, when the switches S1 and S4 are in xe2x80x9cOffxe2x80x9d state and the switches S2 and S3 are in xe2x80x9cOnxe2x80x9d state, through the operational amplifier OP1, the voltage on the node N1 is equal to Vc. When the switches S2 and S3 is in xe2x80x9cOffxe2x80x9d state and the switches S1 and S4 is in xe2x80x9cOnxe2x80x9d state, through the operational amplifier OP1, the voltage on the node N2 is equal to Vc. Thus, the charge-share problem does not occur. The disadvantage of this circuit is that the operational amplifier must work in the wide range of the input frequency and respond quickly to all input frequencies. The result of the charge-share suppression in this circuit completely depends upon the operational capacity of the operational amplifier. A fine design of the operational amplifier is preferred to the result of the charge-share suppression. It is difficult to design such an operational amplifier. Thus, the design of the charge-pump circuit becomes more complex.
To overcome the above problem, Hee-Tae Ahn and David J. Allstet provide a charge-pump circuit for charge-share suppression with transistors (referring to Hee-Tae Ahn and David J. Allstet xe2x80x9cA Low-Jitter 1.9 V CMOS PLL for UltraSPARC Microprocessor Applications, xe2x80x9d IEEE J. Solid-State Circuits, vol. 35, pp. 450-454 March 2000). FIG. 4 is a circuit diagram illustrating the conventional charge-pump circuit for charge-share suppression with the transistors Q1 and Q2. As shown in FIG. 4, when the switch S1 is in xe2x80x9cOffxe2x80x9d state and the switch S2 is in xe2x80x9cOnxe2x80x9d state, a difference in voltage between a source and a gate of the transistor Q1 is VQ1. Thus, the voltage on the node N1 is equal to Vc+VQ1. If the voltage depleted in an impedance of the switch S1 in xe2x80x9cOnxe2x80x9d state is VQ1, the charge-share problem does not occur. When the switch S2 is in xe2x80x9cOffxe2x80x9d state and the switch S1 is in xe2x80x9cOnxe2x80x9d state, a voltage between a source and a gate of the transistor Q2 is VQ2. Thus, the voltage on the node N2 is equal to Vc+VQ2. If the voltage depleted in an impedance of the switch S2 in xe2x80x9cOnxe2x80x9d state is VQ2, the charge-share problem does not occur. Therefore, his circuit has many problems. When the switch is in xe2x80x9cOnxe2x80x9d state, there are two current paths, and it is difficult to detect the current through the switch. Thus, the impedance of the switch in xe2x80x9cOnxe2x80x9d state is difficult to determine, to resolve the charge-share problem. Also, when the current through the switch is small, the impedance of the switch in xe2x80x9cOnxe2x80x9d state must be large. However, in the design of the switch, the impedance of the switch in xe2x80x9cOnxe2x80x9d state must be small. The large impedance of the switch in xe2x80x9cOnxe2x80x9d state cannot be implemented in practical circuit. Finally, the large impedance of the switch in xe2x80x9cOnxe2x80x9d state can be influenced by different procedures and environments. Thus, the design the charge-pump circuit becomes more complex.
An object of the present invention is to provide a charge-pump circuit for charge-share suppression without adding operational amplifiers to decrease the difficulty of design for the charge-pump circuit.
Another object of the present invention is to provide a charge-pump circuit for charge-share suppression with a feedback path to resolve the problems in the conventional charge-pump circuit shown in FIG. 4.
Accordingly, the present invention provides a charge-pump circuit for charge-share suppression comprising a first current source, a first switching element, a first load, a second switching element, a second current source, a second load, a first feedback circuit and a second feedback circuit. The first current source receives a voltage from a voltage generator and provides a current output. The first switching element is coupled between a first connecting node and an output terminal. The first switching element is controlled by an input signal. The first load is coupled between the first switching element and the output terminal. The first load receives the current and outputs an output voltage at the output terminal when the first switching element is in xe2x80x9cOnxe2x80x9d state. The second switching element is controlled by the input signal and opposite to the first switching element. The second current source is coupled between the second switching element and ground and is coupled to the second switching element through a second connecting node. The second load is coupled between the second switching element and the output terminal. The second load receives the output voltage when the second switching element is in xe2x80x9cOnxe2x80x9d state. The first feedback circuit maintains a constant relation between the output voltage and a voltage of the first node, and is not influenced by the status of the first and second switching elements. The second feedback circuit maintains a constant relation between the output voltage and a voltage of the second node, and is not influenced by the status of the first and second switching elements.